# std::ranges::views::as_const, std::ranges::as_const_view

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< ranges::view V > requires ranges::input_range<V> class as_const_view : public ranges::view_interface<as_const_view<V>> | (1) | (since C++23)
namespace views { inline constexpr /* unspecified */ as_const = /* unspecified */; } | (2) | (since C++23)
Call signature |  |
template< ranges::viewable_range R > requires /* see below */ constexpr ranges::view auto as_const( R&& r ); |  | (since C++23)

1) A range adaptor that represents a view of underlying `view` that is also a `constant_range`. An `as_const_view` always has read-only elements (if not empty).
2) RangeAdaptorObject. Let e be a subexpression, let T be decltype((e)), and let U be std::remove_cvref_t<T>. Then the expression views::as_const(e) is expression-equivalent to:

- views::all(e), if it is a well-formed expression and views::all_t<T> models `constant_range`;
- otherwise, std::span<const X, Extent>(e) for some type `X` and some extent `Extent` if `U` denotes std::span<X, Extent>;
- otherwise, ranges::ref_view(static_cast<const X&>(e.base())) if `U` denotes ranges::ref_view<X> for some type `X` and const X models `constant_range`;
- otherwise, ranges::ref_view(static_cast<const U&>(e)) if `e` is an lvalue, const U models `constant_range`, and `U` does not model `view`.
- otherwise, as_const_view{e}.

`as_const_view` always models `constant_range`, and it models the `contiguous_range`, `random_access_range`, `bidirectional_range`, `forward_range`, `borrowed_range`, `common_range`, and `sized_range` when the underlying view `V` models respective concepts.

### Data members

Member object | Definition
`base_` (private) | The underlying view of type `V`. (exposition-only member object*)

### Member functions

(constructor) | constructs an `as_const_view` (public member function)
base | returns the underlying view `V` (public member function)
begin | returns the beginning iterator of the `as_const_view` (public member function)
end | returns the end iterator of the `as_const_view` (public member function)
size | returns the size of the view if it is bounded (public member function)
##### Inherited from std::ranges::view_interface
empty | returns whether the derived view is empty. Provided if it satisfies `sized_range` or `forward_range`. (public member function of `std::ranges::view_interface<D>`)
cbegin (C++23) | returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`)
cend (C++23) | returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`)
operator bool | returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of `std::ranges::view_interface<D>`)
data | gets the address of derived view's data. Provided if its iterator type satisfies `contiguous_iterator`. (public member function of `std::ranges::view_interface<D>`)
front | returns the first element in the derived view. Provided if it satisfies `forward_range`. (public member function of `std::ranges::view_interface<D>`)
back | returns the last element in the derived view. Provided if it satisfies `bidirectional_range` and `common_range`. (public member function of `std::ranges::view_interface<D>`)
operator[] | returns the `n`th element in the derived view. Provided if it satisfies `random_access_range`. (public member function of `std::ranges::view_interface<D>`)

## std::ranges::as_const_view::as_const_view

as_const_view() requires std::default_initializable<V> = default; | (1) | (since C++23)
constexpr explicit as_const_view( V base ); | (2) | (since C++23)

1) Value-initializes `base_` via its default member initializer (= V()).
2) Initializes `base_` with std::move(base).

### Parameters

base | - | a view

## std::ranges::as_const_view::base

constexpr V base() const& requires std::copy_constructible<V>; | (1) | (since C++23)
constexpr V base() &&; | (2) | (since C++23)

Returns the underlying view.

1) Copy-constructs the result from the underlying view. Equivalent to return base_;.
2) Move-constructs the result from the underlying view. Equivalent to return std::move(base_);.

## std::ranges::as_const_view::begin

constexpr auto begin() requires (!__simple_view<V>); | (1) | (since C++23)
constexpr auto begin() const requires ranges::range<const V>; | (2) | (since C++23)

Returns the constant iterator of the view.

1,2) Equivalent to return ranges::cbegin(base_);

## std::ranges::as_const_view::end

constexpr auto end() requires (!__simple_view<V>); | (1) | (since C++23)
constexpr auto end() const requires ranges::range<const V>; | (2) | (since C++23)

Returns the constant sentinel of the view.

1,2) Equivalent to return ranges::cend(base_);

## std::ranges::as_const_view::size

constexpr auto size() requires ranges::sized_range<V>; | (1) | (since C++23)
constexpr auto size() const requires ranges::sized_range<const V>; | (2) | (since C++23)

Returns the size of the view if the view is bounded.

1,2) Equivalent to return ranges::size(base_);

### Deduction guides

template< class R > as_const_view( R&& ) -> as_const_view<views::all_t<R>>; |  | (since C++23)

### Helper templates

template< class T > constexpr bool enable_borrowed_range<std::ranges::as_const_view<T>> = ranges::enable_borrowed_range<T>; |  | (since C++23)

This specialization of `std::ranges::enable_borrowed_range` makes `as_const_view` satisfy `borrowed_range` when the underlying view satisfies it.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_ranges_as_const` | `202207L` | (C++23) | `ranges::as_const_view`, std::const_iterator

### Example

Run this code

    #include <cassert>
    #include <ranges>

    int main()
    {
        int x[]{1, 2, 3, 4, 5};

        auto v1 = x | std::views::drop(2);
        assert(v1.back() == 5);
        v1[0]++; // OK, can modify non-const element

        auto v2 = x | std::views::drop(2) | std::views::as_const;
        assert(v2.back() == 5);
        // v2[0]++; // Compile-time error, cannot modify read-only element
    }

### See also

ranges::as_rvalue_viewviews::as_rvalue (C++23) | a `view` of a sequence that casts each element to an rvalue (class template) (range adaptor object)
ranges::cbegin (C++20) | returns an iterator to the beginning of a read-only range (customization point object)
ranges::cend (C++20) | returns a sentinel indicating the end of a read-only range (customization point object)
as_const (C++17) | obtains a reference to const to its argument (function template)
basic_const_iterator (C++23) | iterator adaptor that converts an iterator into a constant iterator (class template)

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